Structural and base materials · Metals
Cast Iron and Ductile Iron
Explains iron as a cast ferrous material distinct from the rolled and coated steels the library already publishes, and why a cracked casting is commonly replaced rather than repaired.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What cast and ductile iron is
Cast iron is iron carrying far more carbon than steel does, melted and poured into a mould where it takes its final shape as it cools. That single fact separates it from every other ferrous entry in this library. Structural steel is hot-rolled into sections and then cut, bolted and welded into an arrangement; a casting arrives as the shape it will always be, with its wall thicknesses, fillets, bosses and cast lettering formed in the mould rather than worked afterwards.
Two forms are met, and they are routinely confused. In grey cast iron the carbon sits as flakes of graphite dispersed through the metal. The flakes damp vibration and make the material easy to machine, and they also act as internal discontinuities: grey iron is strong in compression, much weaker in tension, and breaks without bending first. In ductile iron, also called spheroidal graphite iron, the melt is treated so the graphite forms as spheres instead of flakes, and the same base material becomes far tougher and able to deform before it fails. Both are cast, both are called cast iron in conversation, and which one is actually present in a building is something to establish rather than assume.
Brittleness is the practical subject. A grey iron casting that is overloaded, struck or dropped cracks rather than bends, and a crack in a casting does not close up again. Neither can it be welded by ordinary site means: the carbon content and the way the metal responds to heating make welding a specialist process with controlled pre-heating and cooling, which is why a cracked gully, cover or rainwater section is commonly replaced rather than repaired.
Corrosion behaves differently from the steels in this category too. Iron rusts, and the rust occupies more room than the metal it came from, so a corroding casting bedded in mortar or built into masonry can split what surrounds it long before the section itself is in difficulty. Castings are protected by coatings applied in the works and maintained afterwards, and the bituminous and protective coating entries in this library exist largely for ferrous metal of this kind.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Cast iron
- Ductile iron
- Grey iron
- Spheroidal graphite iron
- Ironwork
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Rainwater goods — gutters, downpipes, hopper heads and shoes — in traditional construction and in current work where the metal itself is wanted.
- Soil, waste and vent pipework, particularly where the weight and rigidity of the run are part of why it was chosen.
- Drainage ironwork set into surfaces: gully gratings, covers, frames and channel gratings.
- Structural columns, beams and stair components in older buildings, met during alteration rather than specified new.
- Decorative and architectural metalwork including railings, balconies, brackets, ventilators and air bricks.
- Valve, chamber and service-cover components within buried infrastructure.
Consideration
Where it is commonly considered
- Where existing ironwork is being repaired, extended or matched and the replacement has to be the same material as what is already there.
- Where the section depth, rigidity and profile of a casting are part of the character of the building being worked on.
- Where a conservation or planning position makes the original material part of what is being retained.
- Where a casting has cracked and replacement rather than repair is the route being taken.
- Where a ductile iron component is being specified into a loaded or trafficked position as part of a design.
- Where the coating regime for ferrous metal in that position has been thought about before the material is chosen.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where an existing structural casting — a column, a beam, a staircase — is being altered, loaded, notched or exposed, which is engineering work on a material of unknown properties.
- Where nobody has established whether the iron present is grey or ductile, since the two behave differently under load and impact.
- Where a component will be trafficked or loaded, which is a designed and classified question rather than a material choice.
- Where lifting, handling and working at height are involved, because cast sections are heavy and that shapes the whole operation.
- Where existing coatings may be historic materials whose removal or preparation carries health implications.
- Where the casting is built into masonry and corrosion may already have disturbed the construction around it.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Weathering and exposure
- Iron rusts, and the rust takes up more room than the metal it replaced. Protection is therefore a coating regime that is maintained rather than a property of the metal, and the exposure of the position governs what that regime has to be.
- Repairability
- A crack in a casting does not close, and welding is a specialist process with controlled heating rather than a site operation. Replacement is the usual answer, which makes the availability of a matching profile a real question.
- Maintenance
- The coating is the maintained element. Cut edges, drilled holes, site alterations and handling damage are where a maintained surface is commonly first broken.
- Installation dependency
- Weight governs handling, lifting, temporary support, bedding and fixing. What would be a light operation in a formed sheet metal becomes a planned one in iron.
- Appearance
- Crisply moulded profile, mould seams and cast lettering are what distinguish a casting from a rolled or extruded section. Paint sits over it, and the depth and shadow of the profile are what read from a distance.
- Acoustic behaviour
- The mass of a casting and the graphite within it damp vibration, which is why an iron soil stack sounds different in use from a plastic one. How much that matters depends on where the run passes and what it passes through.
- Substrate dependency
- Where ironwork is built into or bedded in masonry, concrete or mortar, the condition of the iron and the condition of what surrounds it are one subject rather than two.
- Surface wear
- Ironwork set into a trafficked surface wears, and a worn casting behaves differently underfoot and under wheels from a new one. What a trafficked component is made to take is a question for the designer and the manufacturer.
Exposure
Exposure and environmental conditions
- Is this ironwork external, buried, trafficked or internal, and what does that do to the coating regime it needs?
- Is the position coastal, industrial, or exposed to de-icing salts, where corrosion advances faster?
- Does water stand in or on the section, as it does inside gutters, hoppers and channel sections?
- Is the iron in contact with another metal, where the behaviour of the junction becomes a subject in itself?
- Is the casting built into masonry, where expanding corrosion can split the construction around it?
- Is it in a position where it will be struck, driven over, or loaded by something nobody has told the designer about?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- Built-in and bedded ironwork depends on the mortar, masonry or concrete around it, and on whether the casting can move as it warms and cools without working against that.
- Brackets, straps and fixings carrying rainwater goods are as much part of the installation as the castings themselves, and they corrode on their own account.
- Jointing between cast sections — sockets, couplings, gaskets, bolted flanges — is specific to the family being used and the components have to match.
- Where iron meets a different metal, separation and the drainage of the junction are design considerations.
- A casting set into a surface transfers its loads through the bedding, the haunching and the construction around it, not through the casting alone.
- In renovation, what the existing connection actually is has to be established by opening up before anything is disturbed.
Appearance
Appearance and finish considerations
- Mould seams, crisp arrises and cast lettering are visible signatures of the forming route and are part of what people are looking at on a period building.
- Paint on ironwork is a maintained surface rather than a finish applied once, and the profile reads through whatever colour is on it.
- A replacement section rarely matches an aged run exactly in profile, in surface or in the way it holds paint.
- Corrosion staining runs down the surfaces below ironwork and marks masonry, render and paving beneath it.
- Where plastic goods have replaced iron on an older building, the loss of section depth and shadow is usually visible from the street.
- Bedding mortar, haunching and made-good surfaces around a casting set into paving are part of what is seen, and they rarely age at the same rate as the iron.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- What coating system is on this ironwork now, and when was it last maintained?
- Are there cut edges, drilled holes or site alterations where the protective system has been broken?
- Is water standing inside gutters, hoppers or sections where it has no way out?
- Is this grey iron or ductile iron, and who has established that rather than assumed it?
- If there is a crack, what caused it — impact, load, corrosion, or movement the casting was restrained from making?
- Is the casting held so tightly by the construction around it that it cannot move as temperature changes?
Upkeep
Maintenance and repair considerations
- How is this ironwork reached for repainting, and what access arrangements does that involve?
- What coating is on it at present, and what does the manufacturer of any new coating say may be applied over it?
- Are matching sections and profiles still obtainable if part of a run has to be replaced?
- Who would identify the metal before any repair is attempted on it?
- Does the existing coating contain historic materials that change how it may be removed?
- Is there a record of which parts of the building carry iron and which carry something else?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Weight governs the operation: handling, lifting, access, temporary support and the number of people involved are all consequences of it.
- Jointing follows the family — socketed, coupled, gasketed or bolted — and mixing components between families is where problems begin.
- Cutting a casting on site is done by trained operatives, and both the dust and the brittleness of the material are part of that.
- Anything set into a surface depends on its bedding, haunching and the surrounding construction to carry load into the ground.
- Damage to the works-applied coating during handling is made good before the item is built in or covered over.
- Support and fixing centres for rainwater goods follow the manufacturer's information for the actual sections supplied rather than whatever the previous run happened to use.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The manufacturer's information for the castings supplied, including the form of iron and the coating applied in the works.
- Whatever the manufacturer states about the duty a component set into a trafficked surface is made for.
- The coating system information, including what may be applied over it during maintenance.
- For existing ironwork, any survey or engineer's report identifying the metal and describing its condition.
- A record of what was removed and what replaced it during renovation work.
- Health and safety information covering cutting, hot work and the removal of existing coatings.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Is this grey iron or ductile iron, and how has that been established?
- If the casting is structural, who is assessing it, and on what basis?
- Is this crack something that can be dealt with, or is replacement the realistic route?
- What coating is on it, what condition is it in, and what can be applied over it?
- How will sections be handled, supported and fixed, given their weight?
- Are matching profiles available if only part of a run is being replaced?
- Does the existing coating contain anything that changes how it may be removed?
- For a component set into a surface, who decides what duty it has to take and who checks that it is what arrived?
Blind spots
Commonly overlooked points
- Cast iron and steel are not the same material under different names. One is poured to shape and one is rolled, and that changes joining, repair and the way each fails.
- Grey and ductile iron look alike in place and behave differently, and most buildings contain both with no record of which is where.
- Welding a casting is a specialist process with controlled heating, not something done with a site set.
- Rust occupies more room than the metal it replaced, which is why corroding ironwork splits the masonry it is built into before the section itself is in trouble.
- The weight is a project constraint in its own right — access, lifting, and how many people are needed to place a section safely.
- Replacing iron rainwater goods with a lighter material changes the appearance of an elevation more than most owners expect.
What this page does not do
- Any structural casting in an existing building — a column, a beam, a stair — is assessed by a qualified engineer before it is altered, loaded or exposed. This reference states nothing about the capacity of any casting.
- Components set into trafficked surfaces carry duty requirements that are project-specific, established with the designer and the manufacturer and verified against local requirements, not taken from a general description.
- Existing coatings on ironwork may contain materials whose removal is a health and safety matter calling for specialist advice before any preparation begins.
- Welding, cutting and hot work on iron castings is specialist work; a cracked casting is commonly replaced rather than repaired, and that judgement belongs to a qualified professional looking at the actual component.
Component forms
Building components commonly formed from this material
Elements this material is commonly used to make, named by the position they occupy in an assembly rather than by what they are bought as. This states common practice only — it is not a statement that this material suits any of these components in a particular building.
Related entries
Common comparisons
Entries met at the same decision but which behave differently, so the comparison is about the difference rather than a swap.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Metals in Construction
Reference entries for the metals used in building — structural and light-gauge steel, galvanised and stainless steel, aluminium, copper, zinc, lead and reinforcement.
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